The invention will be described in the environment of a video signal processing system, however, it is to be understood that it is not limited to such applications.
In video systems recursive filters may be used to reduce noise in the frequency band of the video signal. From frame-to-frame there is a relatively high degree of signal correlation. Thus, if a video signal from successive frames is summed, the correlated video signal will add linearly, but random noise accompanying the signal will not. The summed signal is generally normalized to a desired amplitude range and the signal-to-noise ratio of the averaged signal is enhanced by the processing.
A typical video recursive filter includes a delay device coupled in a recirculating loop with circuitry for combining a fraction of delayed signal with a fraction of incoming signal. The combined signal is applied to the delay device which delays the combined signal by the time period necessary to insure that constituent parts of each combined video signal samples are from corresponding pixels of successive video frames. The fractional parts of the incoming and delayed signals are obtained by scaling the two signals by factors K and (1-K) respectively. Thus, if the amplitude of the incoming signal is equal to the amplitude of the averaged signal from the delay device, the new combined signal will be normalized to equal the input signal. In a digital processing system, the normalization tends to minimize the sample bit size required of the delay device. If the incoming signal consists of 8-bit samples, the sample size in the delay device can be held to e.g. 9 or 10 bits. This is an important design aspect in reducing the manufacturing costs of recursive filters for consumer applications.
When the images represented by video signals change, recursively filtered video signals tend to be degraded. Phantom images are introduced in the reproduced images and picture sharpness along moving image edges is degraded. To counter these undesirable effects it is known to adaptively change the parameters of the recursive filter when motion occurs. The adaptive changes are performed independently for each picture element (pixel). Typically changes are made to the filter scale factors in response to the amplitude of the change in the video signal between successive frames. For larger interframe video signal changes a larger fraction of the incoming signal is applied in the averaging process and for smaller interframe video signal changes a larger fraction of the averaged delayed signal is applied in the averaging process.
After interframe image motion ceases a recursive filter system can be conditioned to more rapidly converge to the desired noise reduction level by applying a sequence of successively smaller scale factors. The timing of the sequence is determined separately for each pixel and is a function of the history of the interframe image motion for the respective pixels. An example of this type of system is described in U.S. Pat. No. 4,240,106 entitled "Video Noise Reduction". The system in this patent develops sequences of scale factors for each pixel wherein successive scale factor values are equal to the reciprocals of the number of frames from the last frame in which motion was detected.
While the foregoing systems tend to enhance the performance of recursive filters in video signal processing systems they do not address an important element in the filtering process. This element is the amplitude of the noise in the signal being processed. If the noise level is low, less noise reduction is required and the filter may be adaptively conditioned to converge at a faster rate than when a greater degree of noise reduction is requred. Programming the system for less noise reduction when the signal noise level is low tends to condition the system to be more tolerant to interframe image motion.
In accordance with the present invention, it is an object to adaptively control a recursive filter as a function of the noise level of the signal to be processed.
Motion detectors for recursive filter systems typically measure the video signal differences between successive frames to determine if the image has changed. Noise attendant these video signals may trigger the motion detector to produce a false indication of motion causing the system to undesirably affect noise reduction. It is a further object of this invention to provide a motion detection system with a low incidence of false motion indications.